A semiconductor layer transfer method called SMART-CUT® is known to those skilled in the art. Details of this method may be found in many published documents, for example, on pages 50 and 51 of: “Silicon on Insulator Technology: Material to VLSI, Second Edition”, “by Jean-Pierre Colinge, published by “Kluwer Academic Publishers”. A second step of bonding the host wafer (the recipient of the transfer layer) typically takes place on the surface of the donor wafer and includes using a bonding layer made of dielectric material such as SiO2. In this manner a semiconductor-on-insulator structure may be formed, such as an SOI structure (in the case where the transferred layer is made of silicon), SiGeOI (when the transferred layer is made of germanium silicon), sSOI (when the transferred layer is made of strained silicon), or GeOI (when the transferred layer is made of germanium).
During the detaching step, thermal energy typically is at least partially utilized. In this case, the thermal budget (the combination of temperature and duration of the heat treatment) must be considered to determine the moment at which the transfer layer will be detached. It has been observed that after detachment of the transfer layer, the latter may have quite a rough surface, as well as a lower quality crystalline surface structure, resulting from the previous implantation and detachment steps.
FIG. 1 illustrates a semiconductor-on-insulator structure 30 that includes a host wafer 20, an electrically isolating layer 5, and a transferred layer 1. The semiconductor part, which is the transferred layer 1, has a reduced crystalline quality surface. In particular, the transferred layer 1 includes a defective zone 1A that has crystalline defects and a detrimental surface roughness. The defective zone 1A typically has a thickness of around 150 nm, and the implantation step may have caused a reduction in the crystalline quality in the transfer layer 1. It is thus necessary to treat the transfer layer 1 to remove the defective zone 1A, and thus to reclaim at least part of the sound zone 1B of the transfer layer 1. Typically, the defective zone 1A is oxidized and then subsequently removed by chemically etching using hydrofluoric acid HF (a treatment called sacrificial oxidation). A finishing step is then used, such as mechanically polishing or chemically-mechanically polishing. Such a treatment step for the transferred layer 1 is costly and complex.
Furthermore, the use of such treatment means requires the systematic removal of the negative of the donor wafer to obtain access to the surface of the transferred layer 1. The wafers must therefore be removed from the furnace (in which heat treatment was conducted). This results in a loss of time, extra wafer handling and the need to use suitable equipment.
Published French application 2,842,349 describes a method that attempts to overcome these problems, by including a stop layer between the future defective zone 1A and the underlying future sound zone 1B. In this example, the transfer layer is made of SiGe and the stop layer is made of Si. This publication also teaches to improve the finishing operations by using selective double etching (of the defective zone 1A and the stop layer), to substantially reduce the roughness as measured from their maximum values (peaks and valleys) and depending on their quadratic values (in RMS Angstroms) on the surface of the SiGe sound zone 1B.
However, selective etching is imprecise, and thus a roughness remains at the tip or edge of the first chemical etching step which is at the interface between the defective zone 1A and the stop layer. Chemical etching therefore unequally treats the surface of the stop layer. In addition, since the stop layer is generally fairly thin, the first selective etching process can pass through it and attack the underlying sound zone 1B. The same document proposes to polish before selectively etching, in order to eliminate some of these potential problems. However, the combination of polishing and selective etching further adds to the complexity and cost of the operation, which could make the overall operation unprofitable.
In published French application 2,842,350 the transferred layer 1 is composed of a sound zone 1B made of strained Si and a defective zone 1A of SiGe. The defective zone is selectively removed from the sound zone 1B. A similar problem exists here analogous to that discussed above, wherein the surface of sound zone 1B is unequally etched because the tip of the etching portion reached it.